With modded 480x I think it's enough, but take into account the efficiency of the supply.

In your example, imagine that you can mod your GPUS, and each modded GPU uses about 140w.

With a 6 gpu rig, you need about 140w x 6 + MOBO aprox  940W.

Well, if your psu is a 80+ gold, that comsuption gets you a 0,88 efficiency, because you are using near a 100% of PSU power.

Half of the power comsuption of your system, gets you a 0,99 efficiency.

In other words, you can save money in your power bill, if you try to use the half part of your PSUs power!

I hope you understand what I'm saying... and sorry for my english!